The amino acid L-isoleucine is essential for man and animal and is widely used as a component of various nutrient mixtures intended for medicinal purposes. Moreover, L-isoleucine is used as an addition for human nourishment and as animal food as well as a reagent for the pharmaceutical and chemical industries.
It is known that L-isoleucine can be fermentatively produced from various carbohydrates such as glucose, fructose, starch hydrolysate, cellulose hydrolysate and molasses or hydrocarbons such as n-paraffins from various precursors of the L-isoleucine biosynthesis. Mainly, members of the genus Corynebacterium, Brevibacterium or Arthrobacter are used for this purpose. They are used in a nutrient solution containing a precursor as well as a carbon and energy source, a nitrogen source, vitamins and other components and which partially convert the precursors to L-isoleucine and accumulate them in the nutrient solution. Threonine, .alpha.-aminobutyrate, .alpha.-hydroxybutyrate or .alpha.-ketobutyrate are used as precursors. These precursors (with the exception of .alpha.-ketobutyrate) can each be present in the L and the D form as well as racemate. The precursors are readily accessible in the form of their racemate, so that the complete utilization of the D and L form must be given for an effective use of precursors.
A maximum L-isoleucine concentration of 15.8 g/l was able to be achieved with Brevibacterium thiogenitalis using 30 g/l hydroxybutyrate as precursor (see U.S. Pat. No. 4,329,427).
This method has the disadvantage that hydroxybutyrate is converted in a small yield to L-isoleucine. However, it is not known to what extent the hydroxybutyrate functioned as precursor and whether the D-component of the racemate of hydroxybutyrate is also utilized.
When D, L-.alpha.-hydroxybutyrate was used as precursor, it was possible to obtain a yield of 78% with Corynebacterium glutamicum ATCC 21193, but only a maximum L-isoleucine concentration of 7.8 g/l (DE-OS 1,910,428).
Even smaller yields can be achieved with the microorganisms of various genera such as, e.g., Corynebacterium, Brevibacterium, Arthrobacter, etc. described in Published German Application DE-OS 1,912,819 (which is equivalent to U.S. Pat. No. 3,671,396).
The small yields are also caused, among other reasons, by the fact that the microorganisms do not utilize the D-.alpha.-hydroxybutyrate.